Re: [PATCH v4 4/6] arm64/kvm: enable pointer authentication cpufeature conditionally

From: James Morse <james.morse@arm.com>
To: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 4/6] arm64/kvm: enable pointer authentication cpufeature conditionally
Date: Fri, 4 Jan 2019 17:58:40 +0000	[thread overview]
Message-ID: <33542a00-23f4-b159-91f5-f05d3afa5b9f@arm.com> (raw)
In-Reply-To: <1545119810-12182-5-git-send-email-amit.kachhap@arm.com>

Hi Amit,

On 18/12/2018 07:56, Amit Daniel Kachhap wrote:
> According to userspace settings, pointer authentication cpufeature
> is enabled/disabled from guests.

This reads like the guest is changing something in the host. Isn't this hiding
the id-register values from the guest?

> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 6af6c7d..ce6144a 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1066,6 +1066,15 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
>  			kvm_debug("SVE unsupported for guests, suppressing\n");
>  
>  		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> +	} else if (id == SYS_ID_AA64ISAR1_EL1) {
> +		const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> +					 (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
> +		if (!kvm_arm_vcpu_ptrauth_allowed(vcpu)) {
> +			kvm_debug("ptrauth unsupported for guests, suppressing\n");
> +			val &= ~ptrauth_mask;
> +		}

I think this hunk should have been in the previous patch as otherwise its a
bisection oddity.

Could you merge this hunk with the previous patch, and move the mechanical bits
that pass vcpu around to a prior preparatory patch.

(I'm still unsure if we need to hide this as a user-controlled policy)

Thanks,

James

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